1. Field of Invention
The present invention relates to a control device of an internal combustion engine.
2. Description of the Related Art
U.S. Pat. No. 4,485,775 discloses an engine having a helically-shaped intake port, which can create a strong swirl motion in the combustion chamber when the engine is operating at a low speed and obtain a high volumetric efficiency when the engine is operating at a high speed. This helically-shaped intake port includes a helical portion, an inlet passage portion tangentially connected to the helical portion, and a bypass passage connecting the inlet passage portion to the helix terminating portion of the helical portion. A swirl control valve actuated by an actuator is arranged in the bypass passage. The actuator includes a vacuum chamber and an atmospheric pressure chamber, these chambers being separated by a diaphragm. This diaphragm is connected to the swirl control valve. The vacuum chamber of the actuator is connected to the intake manifold via a check valve, which permits only the outflow of air from the vacuum chamber to the intake manifold.
When the engine is operating under a heavy load, the vacuum chamber of the actuator is opened to the outside air so that the swirl control valve opens the bypass passage to the maximum extent. Contrary to this, when the engine is operating under a light load; the vacuum chamber of the actuator is disconnected from the outside air and connected to only the intake manifold via the check valve.
When the engine is operating under a light load, a large vacuum is produced in the intake manifold. At this time, since the check valve opens a large vacuum is also produced in the vacuum chamber and, as a result, the swirl control valve closes the bypass passage. The entire air flows into the helical portion from the inlet passage portion of the intake port, and thus a strong swirl motion is created in the combustion chamber.
However, where the diaphragm type actuator is used for actuating the swirl control valve, hysteresis occurs in the operation of the actuator. That is, once the swirl control valve closes, even if the level of the vacuum in the vacuum chamber becomes relatively low, it is possible to maintain the swirl control valve at the closed position. However, to close the swirl control valve, it is necessary to induce a large vacuum in the vacuum chamber of the actuator. In the above-mentioned engine, when the engine load exceeds a predetermined level, the vacuum chamber of the actuator is opened to the outside air, thereby opening the swirl control valve. In this case, even if the level of vacuum produced in the vacuum chamber immediately before the vacuum chamber is opened to the outside air is relatively low, it is possible to maintain the swirl control valve at the closed position immediately before the engine load exceeds the predetermined level.
Contrary to this, in the above-mentioned engine, when the engine load decreases below the predetermined level, the vacuum chamber of the actuator is disconnected from the outside air and connected to only the intake manifold via the check valve. At this time, the level of vacuum in the vacuum chamber of the actuator becomes approximately equal to that of the vacuum in the intake manifold. However, at this time, the level of vacuum in the intake manifold is relatively low and, therefore, the level of vacuum in the vacuum chamber of the actuator is also relatively low. Nevertheless, as mentioned above, a large vacuum is necessary for closing the swirl control valve due to the hysteresis of the actuator. Consequently, in the above-mentioned engine, a problem occurs in that it is impossible to instantaneously close the swirl control valve when the engine load decreases below the predetermined level.
In an engine using an air-fuel mixture having an approximately stoichiometric air-fuel ratio, if the swirl control valve does not instantaneously close when the engine load decreases below the predetermined level, no particular problem occurs. However, in an engine using an extremely lean air-fuel mixture, when the engine load is lower than the predetermined level and using a rich air-fuel mixture when the engine load is higher than the predetermined level, if the swirl control valve does not close when the engine load decreases below the predetermined level, a swirl motion is not created in the combustion chamber when the engine load decreases below the predetermined level. However, when the engine load decreases below the predetermined level, the air-fuel mixture fed into the engine cylinders is changed from a rich air-fuel mixture to an extremely lean air-fuel mixture. Nevertheless, at this time, since a swirl motion is not created in the combustion chamber, the combustion deteriorates. Consequently, when the engine load decreases below the predetermined level, the output torque of the engine is considerably reduced, and thus a problem occurs in that good drivability cannot be obtained.
Of course, it is possible to eliminate such a problem by using an actuator which operates by a small vacuum. However, if such an actuator is adopted, a diaphragm having large size is necessary, and thus the size of the actuator becomes large. However, it is difficult to arrange an actuator having a large size in the engine compartment and, therefore, such an actuator cannot be adopted in practice.